Thyroid hormone mediates a remarkable range of functions in many tissues and organ systems. These functions are important in development and adult homeostasis. The breadth of different tissue responses, including for example in the nervous, sensory and endocrine systems, raises a key question concerning the mechanisms that determine the nature, time and place of a response to thyroid hormone. How does this hormone elicit so many different cellular responses? Thyroid hormone receptors (TR) act as ligand-regulated transcription factors and occupy a key position in the chain of events that elicit the cellular response. Two receptor genes, Thrb and Thra, encode several TR isoforms that are expressed in different developmental and tissue-specific patterns. Thus, the ability to express a given receptor isoform in a particular tissue provides a means of conferring a specific biological response. This project investigates the mechanisms that determine the tissue-specific functions of different TRb1 and TRb2 receptor isoforms encoded by the Thrb gene. The aim is to reveal the developmental and homeostatic functions for these TR isoforms and to investigate other factors that modify the activity of the receptors in different tissues. The project uses model systems to address: 1. Functions of TRb isoforms in differentiation and homeostasis. To determine the biological functions for TRb isoforms, we study mouse models in which TRb1 or TRb2 have been specifically deleted. Our studies demonstrated novel, cell-specific expression patterns of TRb1 in the cochlea and other tissues including the adrenal cortex. In the auditory system, we determined that TRb1 promotes the maintenance of hearing during adulthood and aging. The findings suggest that thyroid hormone may be a factor that ameliorates age-related hearing loss, which is widespread in human populations. In the adrenal gland, TRb1 is expressed in a previously unrecognized cortical cell population and mediates hypertrophic responses to thyroid hormone, suggesting direct actions for thyroid hormone in adrenal gland functions. This cortical cell population is sexually-dimorphic, being transient in males but persistent in females, indicating that thyroid hormone may influence gender-specific differences in adrenal gland function. 2. We investigate additional factors that influence TR activity in tissue-specific fashion. The factors we study include deiodinase enzymes that activate and inactivate thyroid hormone ligand, as well as plasma membrane transporters that mediate the cellular uptake of the hormone into specific tissues from the circulation. In several target tissues (cochlea, retina) deiodinases provide critical control over the activity of TR isoforms. Recent evidence indicates that tissues such as the retina and the testis are subject to control by type 3 deiodinase that degrades hormone to limit the availability of ligand to appropriate levels. The findings support the proposal that specific receptor isoforms and deiodinases function closely together in a given tissue to determine the nature and timing of the response. We have also found that membrane transporters for the uptake of hormone are critical for thyroid hormone action in cochlear development and homeostasis. 3. Target genes for TRb isoforms in natural tissues. To study this critical question regarding the mechanisms by which thyroid hormone stimulates cellular differentiation and function, we investigate the mechanisms of transcriptional regulation of candidate target genes for TRb isoforms. These studies involve a variety of molecular and genomic approaches, to identify changes in gene expression patterns and genomic DNA binding sites for these receptor isoforms in different tissues.